I need some help from the electronics experts on which resistor to use for a standard LED.

NakedMoleRat

Master Member
so, I do a lot of standard wiring of LEDs, but I am by no means an electronics expert. I bought some standard 3 V white LEDs from eBay, shipped from China. They came with a couple hundred resistors as well, but when I wired them up the voltage was dropped so low that they were barely burning. I do not understand the color codes at all, and 20 minutes of very frustrating googling has left me even more frustrated. So, could someone tell me how many OHMS I would need of a resistor to get a nice glow from an LED? That way I can jump on Amazon and order them. As always, thank you for your help.
 

NakedMoleRat

Master Member
So, the ones I used that dropped the voltage too low were 180k ohms, according to the package.

the one I have that works well has three bands, yellow, violet, and the last one is a deep reddish brown? Can anyone tell me what the ohms are for that?

Thank you
 

TazMan2000

Master Member
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You can test out your circuits and change the values and there is a smart feature that simulates activating the circuit, and shows you if it works or not, over/undervoltage.
Great for learning simple electronics and best of all, free.

TazMan2000
 

robn1

Master Member


 

Antsnest

Sr Member
RPF PREMIUM MEMBER
There's 2 key bits of information missing from your question - what voltage is the power source you are using, and what is "a nice glow"?
The first should be fairly easy to answer, but the second is subjective and also a factor of the particular LEDs you bought.

There are two important numbers relating to any 'normal' LED (I'm ignoring 'power' LEDs because these have more complicated driving needs) which are the Forward Voltage (the voltage at which the LED is fully conducting) and the Forward Current (the current it needs to be fully lit when it is fully conducting). These will be specified in the manufacturers data sheet for the specific LED, but frequently this information is not available from 3rd party sellers.

The 3V you mention will be a reference to a typical white LED forward voltage. This is usually 3.2-3.4V but 3V is close enough to be a generic value for simplicity. Also most 'normal' LEDs regardless of colour are designed to have a forward current of around 20mA. Your actual LED may be different, but it will usually be in this ballpark.

For these 2 values the LED will light at a specific brightness, measured in Candellas for small LEDs (Lumens for brighter / bigger LEDs) - this brightness will be determined by the specific LED and will also be provided by the data sheet. This is the figure that can vary wildly from one specific type of LED to another type. Over the last 40 years, the efficiency (and light output) of LEDs has increased by several orders of magnitude. In the 1970's a typical LED had a brightness measured in 10s of milliCandellas (1 mCD = 1/1000th of a Candella) These days, for the same voltage and current rating you can get brightnesses of whole Candellas! But there is still great variation on the market. You can quite easily buy a 3.2V 20mA 300mCD white LED from one vendor and a 3.2V 20mA 3CD from another vendor (or even the same one) for the same price and end up with something that is 10 times brighter than the first when driven by exactly the same circuit.

For any specific LED, to get a brightness other than the rated output, you will need to reduce the current being fed to it down from it's normal operating forward current.

So the short answer to your question is we can't really tell you the "right" answer, you are going to have to do some experimentation yourself. The best approach is to start by working out as best you can what resistor you need to run it at it's nominal forward current - this means the LED is not being overdriven and will be safe from damage. Assuming your LED is rated to run at 20mA (0.02A) you need the following formula (Supply Voltage - 3) / 0.02

Assuming you are using a 5V supply, this will give you a result of 100 i.e. 100 ohms.
For a 12V supply this would give you 450 ohms.

From there, see what brightness that actually gives you. If it's too bright, you need to increase the resistance until the brightness is the 'nice glow' you are looking for. The easiest way is keep adding resistors of the same value again until you reach an acceptable brightness.

Hope this helps!
 

JPH

Sr Member
I bought a buncha LEDs with resistors for my StarLord Helmet.
You only need the *FIRST* one if you wire them end to end.
 

NakedMoleRat

Master Member
I’m using between 9 to 12 V. The calculator showed a 360 OHM resistor, but it was on the ground side. I have always put resistors on the positive side.
 

NakedMoleRat

Master Member
Here are a few pics of the last LED lighting job, to give a reference. Thank you all for your input.
1329C0F5-9C61-44C8-B192-0EAE275355D8.jpeg
7C522A4B-6C83-4A9B-9B80-1F082AD30440.jpeg
E4534E21-7DC2-44DE-A159-CBFB796BD80F.jpeg
 

Bauble

Well-Known Member
“I’m using between 9 to 12 V. The calculator showed a 360 OHM resistor, but it was on the ground side. I have always put resistors on the positive side.” It doesn’t matter. You can put the resistor on either side.
 

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